Portable and/or hand-held machines with electric Rotors are commonly available in a variety of forms and include such machines as washing machines, electric mixers, kitchen appliances and electric tools like electric drills, electric grinders, electric saws, electric cutters operating with grinding blades, circular saws and saber saws, etc.
In the operation of such machines the load upon the electric motor can suddenly or even gradually increase and cause overloading which can result in excessive wear and other detrimental states up to complete breakdown of the machine.
With such overloading, there is even a danger to the user of the machine. For example, in the case of an electric drill, the drill bit may become jammed and the drill body itself may rotate around the drill bit to cause injury to the user.
For these reasons, such machines may be provided with electronic safety systems which prevent such overloads. Such electronic safety systems can be triggered to shut off the machine or reduce the power supplied to it upon the development of an unsafe condition. These systems can be contrasted with systems using a fuse which may burn out or a mechanical safety system with intentional break parts which rupture to prevent injury to the user. In the latter two cases, the fuse or ruptured part must be replaced.
For reliability in the electronic system it is above all important that the system be activated sufficiently rapidly to prevent injury or damage and that the triggering point be fixed with sufficient precision that the operating range in terms of load is not unnecessarily reduced.
It has been found to be a disadvantage of earlier rapidly-operating electronic safety systems that relatively complex means must be provided for determining the load current, thereby making the drill system relatively large and expensive, especially when a high degree of precision is desired. Furthermore, when the load current changes dramatically, i.e. the load current dynamics are significant, there is a tendency for the load current measurement to be inexact which requires that either the trigger point for the cut-off be lowered in terms of load current or that an unnecessarily excessive number of shutoffs be tolerated at least in the critical range.
These drawbacks, together with the fact that earlier attempts to eliminate them have increased the cost of the tool or appliance to an intolerable level or have required place for the control system to the extent that the tool or appliance is no longer sufficiently compact, have resulted in acceptance of less effective control or safety systems than have been desirable.